The differentiation of a B cell to a plasma cell represents one of the most dramatic changes in cellular architecture known. The massive increase in the secretory pathway that occurs is necessary to allow the plasma cell to become a factory dedicated to the synthesis, assembly and transport of immunoglobulin (Ig) molecules. The production of these heteromeric proteins in the endoplasmic reticulum (ER) is both aided and monitored by a group of resident ER proteins known as molecular chaperones. If the Ig protein fails to mature properly, it is identified and transferred back across the ER membrane to the cytosol for degradation by the 26S proteasome. In addition to the massive number of Ig molecules that are processed each minute in a plasma cell, the mechanisms for generating antibody diversity put further demands on ER quality control systems in B lineage cells. Thus it is not surprising that many components of this system were first identified in immune cells, and it is possible that unique elements of the ER quality control apparatus could exist in these cells. To better define the mechanisms governing the biosynthesis of Ig proteins, we continue our studies on the molecular chaperone BiP, which binds to free Ig heavy chains (HC) and prevents their transport until they assemble with light chains (LC). We hypothesize that BiP and its co-factors work together in a carefully orchestrated fashion to aid Ig assembly, monitor the success of this operation, and finally to target improperly folded or assembled Ig subunits for degradation. Furthermore, we hypothesize that distinct regions of the ER exist to accommodate the seemingly antagonistic functions of protein folding and degradation and that individual ERdj family members allow BiP to participate in these different functions. In the present proposal we wish to further delineate critical checkpoints in Ig assembly and determine the mechanisms by which they are executed. To do so, we will determine the function of three ER localized DnaJ homologues in Ig folding, assembly and turnover, define the specificity of nucleotide exchange factors in releasing BiP from unfolded proteins, and finally delineate mechanisms used to identify unassembled Ig molecules and target them for degradation.